Top U.S. intelligence and security officials say Russia’s ramped up presence in Syria should come as little surprise though they remain concerned about Moscow’s increasingly aggressive posture.

“Russia has been very candid. There is some additional people and stuff that is on its way to Syria,” CIA Director John Brennan told a meeting Thursday of intelligence and security professionals in Washington.

“They are stating it is a dual purpose,” he said.

Brennan and other officials say one reason for the influx of troops and supplies is to support the government of Syrian President Bashar al–Assad, a long-time ally, which has suffered a series of setbacks in its battle against Syrian rebels.

The other reason is a growing Russian concern about the growth of the terror group known as the Islamic State.

“We share one of those objectives,” Brennan said, noting he has spoken with Russian counterparts about potentially working together to counter IS.

Moscow-Damascus connection

Military and intelligence officials say, so far, there is no indication any of the Russian forces are operating outside of western Syria, where Russia has long maintained a naval presence.

But Robert Cardillo, the director of the U.S. National Geospatial-Intelligence Agency, said it would be wrong to underestimate Syria’s importance to Moscow.

“It is their touchstone in that region,” he told attendees at the Intelligence and Security and Summit.

Speaking at the same event, Defense Intelligence Agency Director Lieutenant General Vincent Stewart cautioned Russia’s efforts in Syria should not be viewed in isolation.

“Russia is going to be a player on the global stage,” Stewart said. “It’s going to use all of the things in its power.”

Kremlin's explanation

The Kremlin said Thursday that Russian military advisers are in Syria to help that country's armed forces maintain equipment sent from Russia, but are not involved in combat.

Presidential spokesman Dmitry Peskov said the Russian military experts in Syria are involved in "the maintenance of equipment supplied to Syria absolutely within the framework of international law."

He added that the Syrian military is "the only force capable" of countering the advances made by the Islamic State group.

Russian Foreign Minister Sergei Lavrov said Thursday that Russian military personnel have long been present in Syria and are currently helping its armed forces learn how to use Russian military equipment in "the anti-terrorist fight."

He said Russia is sending Syria both military aid in line with existing contracts and humanitarian aid.

Reuters news agency reported on Wednesday that, according to three informed Lebanese sources, Russian forces had started participating in military operations supporting government troops in Syria.

The news agency quoted one of the unnamed sources as saying only a "small" number of Russians were currently involved in these operations.

A senior U.S. defense official told VOA earlier this week that Russia has been airlifting military supplies to Syria, calling such activities “unhelpful.”

State Department concern

On Wednesday, State Department Spokesman John Kirby told reporters that Secretary of State John Kerry had discussed the issue of military aid to Syria by telephone with the Russian foreign minister.

"He reiterated our concern about these reports of Russian military activities — or buildup, if you will — in Syria and made very clear our view that if true and if borne out, those reports would be — could lead to greater violence and more, even more instability in Syria," Kirby said.

White House spokesman Eric Schultz echoed those concerns in his comments to reporters on Wednesday.

"We've made clear that it would be unconscionable for any party, including Russia, to provide any support to the Assad regime,” said Schultz.

Russian aircrafts

In Prague, NATO Secretary General Jens Stoltenberg expressed similar concern about reported Russian deployments of military personnel and aircraft to Syria.

"That will not contribute to solving the conflict," Stoltenberg said. "I think it is important to now support all efforts to find a political solution to the conflict in Syria."

Russia's state news agency Tass on Wednesday quoted a spokesman for the Russian embassy in Tehran, Maxim Suslov, as saying Iran had granted permission for Russian aircraft headed to Syria to fly over Iranian territory.

On Tuesday, Bulgaria refused permission for Russian planes to use its airspace because of doubts about their cargo. On Wednesday, it said it would allow Russian supply flights to Syria to use its airspace only if Moscow allowed the planes' cargo to be inspected at a Bulgarian airport.

Syrian Information Minister Omran al-Zoubi told a television station owned by the Lebanese militant group Hezbollah the reports of increased Russian military involvement in Syria were "concocted in Western intelligence circles.